1. Field of the Invention
The present invention relates to a microcomputer operated with a reset instruction, more particularly a microcomputer in which a reset operation can be properly carried out not only during the rise of a power level, but also the fall of the power level.
2. Description of the Prior Art
For a microcomputer (hereinafter "computer") to start execution of desired jobs, it generally requires a reset operation when power is first applied thereto, especially during the rise of the power. This operation "initializes" the computer.
To achieve such a reset operation, the computer usually incorporates an external reset port. An external reset circuit, such as a capacitor-resistor (CR) circuit, is connected to the pin leading to the external reset port.
A recent trend is for execution of a similar reset operation when the power is cut off, especially during the fall of the power level. Erroneous operation often occurs during the fall of the power level, especially when the level drops below the lower limit of a guaranteed operational range of the computer, for example 4.5 V (5 V-0.5 V). Specifically, the computer may jump to a totally different routine and run out of control.
The prior art has used both conventional and additional external reset circuits for the reset operation in order to cover both the rise and fall of the power level. That is, computers have been equipped with both conventional and additional external reset circuits at the appropriate pin. The external reset circuits in current use compare in size with the computer itself, however. This accordingly increases the size of the computer assembly. With very small systems, for example, an electronic camera system, there is not much room available of the electronic components.